With an aging population (about age 40 and older), a need exists for the rapid and efficient correction of vision with a single device for both near and far distances. An aging person often may be afflicted with presbyopia, alone or in combination with myopia and hyperopia, and the impairment of vision due to advancing years. Presbyopia results in a diminution of the power of accommodation from loss of elasticity of the crystalline lens of the eye (and the change in the shape of the lens), causing the near point of distinct vision to be removed further from the eye. Hyperopia is described as that error of refraction in which rays of light entering the eye parallel to the optic axis are brought to a focus behind the retina, causing blurred vision. Hyperopia is a result of the eyeball being too short from front to back; the condition is also called farsightedness. Myopia is described as the error of refraction in which rays of light entering the eye parallel to the optic axis are brought to a focus in front of the retina as a result of the eyeball being too long from front to back; the condition is also called nearsightedness.
A number of methods have been used to treat presbyopia, alone or in combination with hyperopia and myopia. These include two or more complete sets of ophthalmic glasses (spectacles); for example, one set for distance vision and another set for near or reading vision. An additional method uses half-size reading glasses ("granny glasses") to correct for the near vision while the distance vision remains normal or uncorrected. The most common method of correction of presbyopia is the use of bifocal or multifocal spectacle eyeglasses.
A number of methods employing contact lenses have been used to correct near and distance vision, for example, the use of a near vision contact lens on one eye and a distance vision contact lens on the other eye (monovision), concentric annular bifocal contact lenses, fused bifocal contact lenses, variable front surface power contact lenses, a combination of distance vision contact lenses with half vision spectacle reading glasses, and the like. Additional methods, including the use of bifocal contact lenses, are described in Contact Lens Practice, Third Edition, by R. B. Mandell, published by Charles C. Thomas of Springfield, Il., in 1981, particularly pp. 704-731, and in a number of patents, e.g., U.S. Pat. Nos. 3,031,927; 3,279,878; 3,950,082; 4,618,229; 4,199,231; 4,318,595 and 4,324,461; and U.K. Pat. No. 2,033,101.
Bifocal contact lenses have typically had two distinct optical zones within a single lens. In a hard bifocal contact lens, the different optical zones have often been achieved by fusing together materials having different indices of refraction. Another method of creating different optical zones is by cutting differently curved segments (zones) on the surface of the lens.
In a translating bifocal contact lens, the lens must translate the vertical distance of one optical zone over the pupil of the eye so that the wearer can see through the other optical zone. With certain previous lenses, the large adhesive forces of the lens to the cornea and sclera (the bulbs conjunctiva) precluded the necessary reliable translation.
With the recent development of the plastics industry, it has been found that methacrylic ester polymers alone or those obtained by copolymerizing a methacrylic ester, such as methyl methacrylate, with other comonomers have superior properties of phtotransmission, density, stability, oxygen permeability and the like, and have been used extensively as contact lens materials.
Hard lenses are usually produced using the polymerized monomer, methyl methacrylate. The poly(methylmethacrylate) contact lens is essentially nonpermeable to oxygen, so corneal edema (swelling and clouding of the cornea) has often been a problem when these contact lenses are used. These hard lenses are usually small, on the order of about 7.5 to 9.5 mm in diameter and are held to the cornea of the eye by the forces of the tear layer. A small optically decentered bifocal contact lens was reported by C. M. Taylor in The Optician, Vol. 149, pp. 287-8 (1965). The lens described is a hard lens having a maximum overall diameter of 9.7 mm.
Rigid, gas permeable contact lenses are made from materials including polymerized comonomers such as cellulose acetate butyrate (see U.S. Pat. No. 3,900,250) or acrylate-silicone esters (see, for example, U.S. Pat. Nos. 3,080,178 [Re. 31,406] and 4,216,303). These rigid, gas permeable lenses, as the term implies, are permeable to oxygen in varying degrees, thus reducing or eliminating corneal edema. These rigid lenses are also small, on the order of about 7.5 mm to 9.5 mm in diameter.
Neefe (U.S. Pat. No. 4,239,712) describes a bifocal contact lens wherein the near vision segment is an upturned crescent cut on an eccentric lathe while protecting an already-polished distance segment by coating it with a protective polymer layer. The Neefe contact lens, however, only provides for prism to be incorporated into the anterior or the posterior curves, lacking the weight distribution and optical effects from combining prism offered by the present invention's use of prism in both the base curve and in the distance vision curve. Thus, a compound prism is obtained in the distance segment of the present invention (e.g. if there are 1.75 diopters of prism in the base curve and 1.25 diopters of prism in the distance segment front curve, the distance vision zone would offer effectively 3.0 diopters of prism, while the near vision segment would have the 1.75 diopters of prism from the base curve only.) Another feature distinguishing the lens of the present invention and the Neefe lens is the effect of the lenticular zone combined with the above-described prism; use of a lenticular zone is very effective in distributing mass and allows more options for selecting and controlling edge thickness. Also the lens of the present invention has uniformly thickening edge (from apex to ballast), adding to the comfort of the lens and improving its ability to translate. The Neefe lens employs a small rim portion to add ballast and to prevent the near segment from sliding beneath the lower lid in the reading position, but does not offer the uniformly thickening edge and variety of lens parameter permutations which are available with the lens of the present invention.
A protective coating technique is also taught in U.S. Pat. No. 3,300,909 to Cooper, et al., but there, prism is only provided in the distance curve; the combined prism, lenticular zone and uniformly thickening edge of the present invention are not shown or even suggested.
None of the prior art bifocal contact lenses, hard or hydrogel, are free from all adverse physical and optical effects, such as low oxygen permeability, irritation, severe optical jump, blurring, simultaneous vision effects (double vision), and the like. Thus, prior attempts to solve the presbyopia problem using the aforementioned techniques cannot be said to be ideal for all wearers.
It would, therefore, be valuable to have a small, rigid, gas permeable bifocal contact lens, which will reliably translate vertically up and down on the cornea. Reliable translation requires that the portion of the pupil covered normally by the distance vision optical zone be substantially covered by the near vision optical zone when required. Thus, the wearer achieves good vision in both optical zones with a minimum of physical and optical side effects.